1. The Field of the Invention
The present invention relates to the field of optical communications. More particularly, the present invention relates to systems and methods using diffractive and optionally refractive surfaces to launch an optical signal into a multimode optical fiber with a controlled spatial intensity and phase profile.
2. The Relevant Technology
Computer and data communications networks continue to develop and expand due to declining costs, improved performance of computer and networking equipment, the remarkable growth of the internet, and the resulting increased demand for communication bandwidth. Such increased demand occurs within and between metropolitan areas as well as within communications networks. Moreover, as organizations have recognized the economic benefits of using communications networks, network applications such as electronic mail, voice and data transfer, host access, and shared and distributed databases are increasingly used as a means to increase user productivity. This increased demand translates into a need for higher speed communications links for which fiber optics is particularly well suited.
Through fiber optics, digital data in the form of light signals is formed by light emitting diodes or lasers and then propagated through a fiber optic cable. Such light signals allow for high data transmission rates over distances for which electrical links are poorly suited. Other advantages of using light signals for data transmission include their resistance to electro-magnetic radiation that interferes with electrical signals; fiber optic cables' ability to prevent light signals from escaping, as can occur with electrical signals in wire-based systems; and light signals' ability to be transmitted over great distances without the signal loss typically associated with electrical signals on copper wire.
One important device for fiber optic communications is the laser. Generally, a laser is a light source that produces, through stimulated emission, coherent, near monochromatic light. The emitted laser light can be modulated to provide optical signals that can be transmitted over great distances. In this manner, an electrical signal is converted to an optical signal for data transmission. The optical signal is, in turn, received and converted back to an electrical signal by a receiver such as a monitor photodiode. A transceiver is an optical device that includes both a laser (as part of a transmitter) and a photodiode (as part of a receiver).
The optical signal can be coupled to and from either multimode or single-mode fiber. The term “mode” refers to an electromagnetic wave traveling in a fiber or other waveguide which has a particular spatial field and phase distribution and which travels at a characteristic velocity. A small core optical fiber, for example 8-9 microns, can carry only a single-mode and is therefore termed single-mode fiber. Such a fiber is well suited for large transmission distances because all of the light travels with a well defined velocity. A larger core diameter fiber, for example 62.5 microns, can propagate more than one mode of light and is therefore termed multimode fiber. Multimode fiber is best suited to shorter transmission distances, for example within local area networks systems, while single-mode fiber is best suited to longer transmission distances such as long-distance telephony and cable television systems.
Single mode fiber has advantages in that single-mode fiber allows for a higher bandwidth-distance capacity to transmit information because it can retain the fidelity of each light pulse over longer distances and it exhibits no dispersion caused by velocity differences between multiple modes. Single-mode fiber also enjoys lower fiber attenuation than multimode fiber. Thus, more information can be transmitted per unit of time. As a result, single mode is often preferred for optical communications. Nevertheless, multimode fiber has some advantages that caused it to be used in a large number of shorter distance applications, generally less than 2 km and usually less than 500 m. Such distances form the majority of connections in local area networks (LANs) and similar storage area networks (SANs). First, earlier fiber optic links at relatively low data rates of 100 Mb/s or less were based on very low cost LED (light emitting diode) sources. The highly multimode nature of a typical LED's output makes it impossible to couple a useful portion of the light into a single mode fiber. Thus, relatively large core (50-62.5 um diameter) multimode fibers were used to collect a reasonably larger fraction of the LED output. Additionally, and perhaps even more importantly at the time, the mechanical tolerances needed to make fiber optic connectors and the corresponding receptacles on fiber optic transceivers for single mode fiber were very expensive and made them impractical for the cost sensitive short data links.
For these reasons, multimode fiber became, and to a large extent remains, the practical standard for fiber optic cabling within typical office buildings and the like. This has led to a large infrastructure of legacy multimode fiber and a corresponding desire to use this fiber for newer higher speed links.
One of the limitations on the bandwidth distance product of a multimode fiber link (that is the maximum data that can be transmitted over a given link distance), is caused by differences in propagation velocity of the different modes of a multimode fiber. These differences, usually referred to as modal dispersion, cause a pulse representing a single data bit to spread in time and interfere with adjacent pulses causing what is known as ISI or inter-symbol interference, which will degrade the quality of the received signal and ultimately cause a link to become unusable. In an ideal multimode fiber, the differences in modal velocity are very small and the bandwidth distance product is actually very high (as high as 10 GHz*km). In real fibers, however, manufacturing imperfections in the refractive index profile of the fiber lead to a much larger range of modal velocities and limit the modal bandwidth to 160-500 MHz*km depending on factors such as the optical wavelength used. One particularly common imperfection of multimode fiber index profiles is a significant peak or dip in the refractive index at the center of the fiber core, which tends to lead to large differences in the velocity of the lowest order modes relative to the remaining modes.
As the need for higher data rates in LANs and SANs increased, the problems of modal dispersion became a significant limitation. This limitation became a particular problem during the development of Gigabit Ethernet (which uses signaling rates of 1.25 Gb/s) in 1998, and threatened to significantly limit the distances. See DAVID CUNNINGHAM ET AL., GIGABIT ETHERNET NETWORKING (June 1999), incorporated herein by reference. The solution adopted for Gigabit Ethernet involved controlling the optical launch into the multimode fiber to avoid launching into modes that lead to particularly low bandwidth.
One approach to reducing modal dispersion in multimode fiber is to launch a small spot off-center into the fiber. This launches a reduced set of optical modes, and in particular tends to avoid modes which are strongly affected by imperfections in the fiber core as well as at the edges of the optical fiber. This is commonly performed by a “mode-conditioning” patch cable external to the fiber optic transceiver where the transceiver provides a single mode launch and the mode conditioning patchcord consists of a length of single mode fiber joined to a length of multimode fiber with the desired lateral offset. Unfortunately, this is a relatively expensive solution requiring additional hardware that adds an incremental cost to the optical system in which it is employed and requires an increased degree of care by the end user to make sure the patchcord is employed in links that require it.
Another approach to conditioned launch is to launch light in a ring shaped pattern where the intensity is small at the center and edges of the fiber. This general approach was standardized during the development of 10 Gigabit Ethernet to ensure usable distances on links based on 850 nm multimode laser sources. This approach specified a test of the optical power distribution at the entrance to the span of multimode fiber which set an upper bound on the power within a particular small radius and a lower bound on the power within a much large radius.
At the time of this writing, a standard is under development to allow transmission of 10 gigabit/second signals over legacy multimode fiber links with distances of up to 300 m. In this case, even with good control of the launch into the multimode fiber, the ISI is still too large for simple receivers to function well. The solution being developed in this standard is to use electronic dispersion compensation techniques in the receiver of the system to compensate for the ISI. As it turns out, even with practical degrees of electronic dispersion compensation, careful control of the launch into the fiber is still needed, and more significantly, the criteria for what is an acceptable launch is somewhat different than in the case of simpler receivers.
Another prior art approach to launching an optical signal into an optical fiber includes adding a discreet insert to a transmitter or transmitter optical subassembly (TOSA). In this approach, an optically transparent insert with a diffractive pattern is placed in the optical path in the TOSA. The diffractive element launches the optical signal with a controlled intensity and phase distribution. One particularly useful launch is one where the amplitude distribution is in a donut form and the phase varies periodically around the azimuth of the fiber. Such a launch will excite modes which travel in a helical trajectory, when thought of in the ray picture, and are particularly well suited for avoiding imperfections in the core of the optical fiber. This launch has been referred to in the literature as a “vortex launch,” and an optical element which generates this launch as a “vortex lens.” Further details regarding a “vortex launch” and similar methods are disclosed in U.S. Pat. No. 6,530,697 B1, filed Jun. 11, 1999, U.S. patent Publication No. US2003/0142903 A1, filed Nov. 12, 2002, and U.S. Pat. No. 6,349,159 B1, each of the foregoing being incorporated herein by reference in their entireties. More generally, the term “vortex launch” is used herein to refer to any launch which results in a substantially spiral or helical propagation in the optical fiber. The term “vortex lens” is used herein to refer to an optical element such as a diffractive surface which will generate a vortex launch. This approach has several distinct disadvantages, however. For example, the fabrication of an additional part adds an incremental cost to the TOSA. In addition to the cost of fabricating the part, there is an assembly cost in that the discreet diffractive grating part has to be carefully aligned during assembly of the TOSA. Optical systems are a very competitive industry and each such cost decreases the competitiveness of a product.
Accordingly, it would represent an advance in the art to provide less expensive methods and systems to reduce modal dispersion in existing multi-mode fibers and thereby improve the transmission of single mode optical signals over multimode fiber.